1. Technical Field
The present invention relates to an image inspection apparatus to inspect images formed on sheets of recording media by an image forming apparatus such as a printer, a multi-functional apparatus using image data prepared from print data, an image forming apparatus provided with the image inspection apparatus, an image inspection method, and an image inspection program.
2. Description of the Background Art
Printers or the like form images on recording sheets based on print data. To produce high-quality printed images, the printers may be provided with image inspection apparatuses to inspect the output images.
Conventional image inspection apparatuses use an image scanner to scan images printed on sheets of paper, etc., and compare the scanned images to a model image prepared in advance. For example, in the case of offset printing machines, which use the same printing plates to print a large number of copies, image inspections are conducted by scanning a high-quality image with an image scanner of an image inspection apparatus and using the scanned image as the model image.
At present, such image inspection capabilities are also desired for printers used for on-demand printing, in which print data may be different for every sheet and consequently it is not realistic to prepare model images. It is possible to generate a model image when a print request is issued with print data, using, for example, a configuration that uses image data from original image data (print data) that has not been scanned as a standard for comparison and inspecting target printed images, which is disclosed in JP-2007-148027-A.
In conventional image inspection, the image scanner for scanning inspection of target printed images is a sheet-through type, in which a document sheet printed with an image thereon is transported in a certain direction and an optical sensor having an opening disposed at a given position along the transportation route detects light reflected from the document sheet to scan the printed image. The opening is fitted with a contact glass that contacts the document sheet, and thus the distance between the optical sensor and the document sheet can be kept constant by the contact glass. Further, a reference white plate or background scanning plate is disposed opposite the contact glass via the document transport route that also guides the document sheet. With such a configuration, the document sheet may contact not only the contact glass but also the reference white plate. Further, because the inspection target printed image has just been printed, the ink may not be completely dry by the time image inspection is conducted. As a result, the contact glass and the reference white plate may be stained with ink where they contact the document sheet. Stains on the contact glass may get mixed in with the scanned image and appear as noise, thereby degrading scanning precision.
If inspection is conducted under such conditions, it becomes difficult to determine whether a poor inspection result is caused by the poor quality of the inspection target printed image itself or by stains on the contact glass, by which a false inspection or miss-inspection may occur.
Further, because the reference white plate is used as a standard for shading correction in image scanning, stains on the reference white plate may also get mixed with the scanned image as noise. If shading correction is not conducted effectively due to such stains, both image scanning precision and inspection precision deteriorate.
Such false inspection or miss-inspection and poor inspection precision may be prevented by removing stains on the contact glass and the reference white plate periodically, or by displaying a stain removal prompt when the detected light intensity of the reference white plate drops below a critical threshold and removing the stain.
Such periodic stain removal can be conducted by monitoring by a user such as an administrator. To conduct such removal, operation of the image inspection apparatus and/or image forming apparatus must be stopped, but if for some reason the image inspection apparatus and/or image forming apparatus cannot be stopped immediately, such stain removal may not be conducted at an appropriate timing.
Further, because the reference white plate is scanned through the contact glass, the low intensity of detection light coming from the reference white plate may be caused by stains on either the contact glass or on the reference white plate; i.e., the stain origin cannot be identified. Therefore, the image scanner must stopped and stains removed from both the contact glass and the reference white plate, and after confirming that the detection light intensity of the reference white plate again achieves a desired value, the image scanner must be activated again.
If the stain origin is not the contact glass but the reference white plate, the image scanner may not be required to be stopped immediately and the reference white plate may not be required to be cleaned because image inspection can be conducted without too much degradation of image inspection precision.
If the image inspection can be conducted uninterrupted, the processing performance of image inspection can be enhanced. However, as above mentioned, because the stain origin cannot be identified when the reference white plate is scanned in the conventional method, such enhanced image inspection is not practical.